The International Telecommunications Union is responsible for assigning various blocks of letters and/or numbers to all the countries on Earth for use in callsigns. For example, Nauru is the only country that can use call letters beginning with C2; France gets to use FA through FZ, HWA through HYZ, TH, TK, TM, TOA through TQZ, and TVA through TXZ. (Many of the other countries holding a portion of the T's are former French colonies in Africa.) Beyond the initial letters or numbers, it's up to each individual government how they want to divvy up callsigns.

Very few countries require broadcasters to use their callsigns on the air frequently; in many cases, it's only at sign on and signoff, and sometimes not even then. However, the Federal Communications Commission in the U.S. requires broadcasters to announce their call letters and city of license once an hour, and as such, radio and TV stations there tend to be known more by their call letters than anything else.

After the first letter, the next three letters in the sequence can be chosen by the licensee, although they were assigned in sequence in the early days (again, KDKA is an example of this). Some older stations have only two letters after the W or K, such as WBZ in Boston, KHJ in Los Angeles, and another Pittsburgh radio station that's breaking two current rules, KQV.

In most cases, the person doing the call letter picking tries to come up with a memorable set, since the station is going to have to use them to identify itself anyway. There are plenty of ways to do this:

At least since the 1970s, many American radio stations have tried to deemphasize their call letters, instead primarily identifying themselves by a different name, usually including the frequency. However, many of these will either be based on the existing call letters anyway, or will result in a call letter change when the name changes. There are a ton of radio stations calling themselves "Kiss FM," for example, but many have call letters such as KIIS or WVKS.

The same thing has been happening in television since the late 1980s, when the Fox network persuaded many of their affiliates to refer to themselves as "Fox (channel number)." This idea has since been copied by the other networks and their affiliates.

However, not only are the FCC regulations still in effect requiring stations to identify themselves by their call letters once an hour, more importantly, both the Nielsen television ratings and the Arbitron radio ratings primarily use the call letters to determine who was watching or listening to what. If a misguidedviewer in Los Angeles writes in a Nielsen diary that he was watching "7 News" on NBC 4, KCBS, the ratings will go to Channel 2, KCBS. More than anything else, that assures the future of call letters in the U.S.